Electrostatic photographic printing



United States Patent O' -2,758,525 ELECRosTATIc PHoTooRAPHrc PRINTING Alexander J. Moncriel-Yeates, 'Cranbury, N. J., assignor to Radio 'Corporation of America, a corporation of Delaware Application December 30, 1953, Serial No. 401,189 5 Claims. (Cl. 951.3)

The electrostatic image is developer powder which is areas of the sheet.

rendered visible by applying a held electrostatically to charged The powder image thus formed may For example, utilizing apparatus for giving the photoconductive surface an over-all charge and apparatus for projecting an optical image on the charged surface. When the electrostatic image has been formed, additional apparatus is employed to render the electrostatic .image visible.

Because these steps are separate and -distinct operations, the photoconductive layer must have the ability to store the electrostatic image at least for the period of time necessary to form and to develop the electrostatic image. By combining the two steps in a single operation, photoconductive materials having very short storage times, that were hitherto impractical, may be used. It is also cornmercially desirabe to combine these steps into a single operation in order to speed up the printing process and simplify the required apparatus.

Heretofore used electrostatic printing processes are capable of producing either a positive or a reverse visible image in a single operation, but not both. Whether the image is positive or reverse is determined by areas whether the illuminated or non-illuminated areas of the electrostatic image-bearing sheet are the charged areas, the polarity of the charged areas, and the polarity preference of the developer powder.

An object of this invention is to provi-de improved methods and means of electrostatic printing.

Another object is to provide a single, simplified electro- 2,758,525 Patented Aug. 14, 1956 static printing apparatus for accomplishing the steps of forming and rendering visible an electrostatic image.

Another object is to provide methods and means of electrostatic printing that may use photoconductive materials having relatively short storage times.

A further object is to provide methods and means of electrostatic printing for producing both a positive and a negative visible image in the same operation.

in accordance with the present invention, a visible powder image may be produced by lirst placing a thin layer of developer powder, such as carbon black, upon the upper surface of a photoconductive insulating layer, such as a layer of photoconductive insulating selenium; then simultaneously applying a unidirectional electric eld to said developer powder layer and projecting a light image upon said photoconductive insulating layer. Simultaneous application of the electric eld and projection of the light image charges the developer powder lying upon the illuminated areas of powder away from the photoconductive layer leaving a rst visible powder image thereon. Another visible powder image, which is the reverse of the foregoing Visible powder image, may be produced by intercepting the charged developer powder attracted away from said photoconductive insulating layer upon an electrically-insulating surface in closely spaced relation to said photoconductive insulating layer.

The invention will be more easily and fully understood from the following detailed description in conjunction with the accompanying drawing in which:

Figure l is a partially schematic sectional elevational view of a camera embodying this invention, and

Figure 2 is a sectional elevational View of the electrophotographic pack that may be used in the camera of Figure 1.

Figure 3 is a sectional elevational view of a second electrophotographic pack that may be used in the camera of Figure l,

Figure 4 is a sectional elevational view of a third electrophotographic pack that may be used `in the camera of Figure l.

Similar reference characters are applied to similar elements throughout the drawing.

Example l Referring to Figures l and 2, an electrophotographic pack 79, which may be utilized in the present invention, comprises an upper brass electrode 27 spaced about 1/8" above a lower electrode by means of spacers 67. The lower electrode comprises a transparent backing plate 21, such as glass, a transparent electrical conductive layer 23, such as NESA, marketed by the Pittsburgh Plate Glass Company, superimposed thereon, and a coating of a photoconductive insulating material 25 such as selenium superimposed upon the transparent coating 23. A thin even layer of developer powder 4l such as carbon black is placed on the upper surface of the photoconductive layer 2S. The powder may be applied, for example, by sprinkling the powder from a suitable shaker before the electrophotographic pack 79 isI assembled. A camera is provided comprising a lens Sl, a lens holder $3, a shutter S4, a shutter mechanism 85, an enclosure 87, a pack holder 89 and a spring clip 9i. The electrophotographic pack 79 is placed in the camera by means of a hinged door 93 and held in place by means of a pack holder 39 and a spring clip 91. The upper electrode 27 and the transparent coating 23 of the lower electrode are connected to a voltage source 6l through double-pole, doublethrow switch 63 and a potentiometer 65.

A negative voltage of about 400 Volts is applied to the upper electrode 27 by throwing the switch 63 and adjust ing the potentiometer 65. The shutter 84 is opened by means of shutter mechanism 85 allowing the image of an object 99 to fall upon the photoconductive coating 23 of the lower electrode. The electrical resistance of the illuminated areas of the photoconductive coating 23 is reduced due to the presence of light, allowing an electric charge to move to the upper surface of the photoconductive coating 23, thus forming an electrostatic image corresponding to the illuminated areas of the electrode. The developer powder 41 resting on the charged areas (illuminated areas) of the photoconductive coating 23 becomes charged in the same polarity as the area upon which it is resting. The powder thus charged is attracted upwards due to the electric field present including the relatively strong field existing between the upper electrode 27 and the transparent conducting coating 23 of the lower electrode, leaving behind a first visible powder image on the photoconductive coating 2S of the lower electrode. The powder attracted upwards cornes to rest on the lower surface of the upper electrode 27, forming a second visible powder image that is the reverse of the first powder image. The electric field is disestablished by opening the switch 63 and the electrophotographic pack is removed from the camera. The visible powder images may be fixed to the surfaces upon which they are resting or they may be transferred to another surface and fixed thereon by any of the conventional methods. For example, an adhesive may be sprayed on the powder image, or, if the powder is fusible such as a colored vinyl chloride resin, it may be fused to the surface upon which it rests by the application of heat.

Almost any photoconductive material may be used to makeV the photoconductive coating 23. Examples of other photoconductors are cadmium sulphide, anthracene, and antimony trisulphide. Mixtures of a photoconductor and up to about 50% of a dielectric film-forming material such as a synthetic resin may also be used. The photoconductive layer may be produced by any convenient method, for example, brushing, spraying or evaporating the material upon a surface of a substrate. Many electrostatic printing processes require the use of photoconductive materials having relatively long relaxation times. The relaxation time of a photoconductor is a measure of its ability to store an electric charge and is commonly expressed as the'period of time in which an arbitraryper cent of a charge stored on its surface will leak off. One of the advantages of the present invention is that the photoconductive materials that may by their relaxation times because the photoconductive material is not required to store a charge.

The photoconductive material must be sufficiently insulating to prevent a build up of charge on its surface in the darkness while the voltage is applied to the electrodes. Since the voltage is applied for only a very short period of time, ordinarily photoconductive materials with very short relaxation times may be used. The switch 63 may be synchronized with the shutter 84 so that the electric field is established for only a fraction of a minute and'therefore photoconductive materials that are sometimes considered conducting may be used.

While any type electromagnetic radiation may be projected upon the photoconductive layer, the spectral range of the radiation must be within the range of sensitivity of the photoconductive material. Thus the choice of photoconductive material and radiation are dependent upon one another.

Any powder may be used to produce the visible image by the method of this invention. Materials that have a relatively high electrical conductivity become charged and attracted upwards more rapidly. However, relatively insulating powders work equally well although longer exposures are required. lt is preferred to use powders with particle size ranges between l and 100 microns. Surface forces interfere with the process when the powder is too fine. Particles that are toocoarse are` difhcult' tobe used are not limited 4; handle and tend to lose definition in the image. Examples of suitable developer powders are sawdust, carbon black, brass, magnesium silicate and polyvinyl chloride resins. The powders may be stained with suitable dyes.

The spacing of the electrodes and the applied voltage are not at all critical. It is only necessary to provide an electric field which, when added to the smaller field, due to the electrostatic image, will cause the charged powder to move upwards. Electric fields of the order of 3 to l0 kilovolts per inch are preferred for the specific arrangement of Example l. The preferred range of field strengths in other cases will be determined in part by the arrangement of the elements and the nature or the materials used.

In the arrangement of Example l, the powder moving upwards comes to rest on the lower side of the upper electrode 27. lt is held here by surface forces and by electrostatic forces due to the difference in polarity between the powder and the electrode. The powder particles will eventually discharge and then become charged in the polarity of the upper electrode. The combined effect of gravity and the repulsive effect between the powder and the upper electrode may overcome the surface forces and the powder particles will return to the lower electrode. It is therefore desirable to remove the electrophotographic pack and place the image bearing surface of the upper electrode upwards to prevent the loss of the image.

Referring to Figure 3, another arrangement may be used to prevent the loss of the image on the upper electrode. The apparatus is similar to the apparatus of Figure l except that the upper electrode is coated with an insulating layer 29. The coating 29 may be any electrically insulating material such as synthetic resins or paper. The layer may be a coating as shown or it may be a sheet placed close to but spaced from the upper electrode. The insulating layer prevents the charged particls that come to rest thereon from discharging. In this way, the charged powder particles that come to rest on the surface of the insulating layer are held tightly by electrostatic forces for a greater period of time than in Example l.

Referring to Figure 4, a third arrangement may bc used to prevent the loss of the image on the upper electrode. The apparatus is similar to the apparatus of Figure l except that a carrier sheet 3l having on its lower surface a coating of a sticky substance 33 is placedv against the lower surface of the upper electrode 27'. The powder particles' moving upwards come to rest in the layer of sticky substance 33 where they are held. The sticky substance may be coated directly onthel lower surface .of the upper electrode. Examples of suitable sticky substances are gelatin, gum arabic,l pasted plastics, common plasticised adhesives and petroleum jelly.

There have been described improved methods and means of electrostatic printing. T he apparatus is simple in construction and rapid in operation. Using the method and' apparatus of the invention, both positive and reverse images may be formed in the same operation. Photoconductive materials having relatively short storage times that were hitherto unfeasible may now be used.

What is claimed is:

l. A method of electrostatic printing comprising first placing a thin layer of developer powder upon the upper surface of a photoconductive insulating layer supported upon a lower electrode and spaced from an upper electrode; then simultaneously applying a unidirectional electric field betweensaid electrodes and projecting a light image upon said photoconductive insulating layer to charge the developer powder lying upon the illuminated areas of the photoconductive insulating layer and to attract the charged powder away from the photoconductive insulating layer leaving a first visible powder image thereon.

2. A methodv according to claim l including intercepting the charged developer powder attracted away from said photoconductive insulating. layer upon an electrically-insulating surface in closely spaced relation to said photoconductive insulating layer to produce a second visible powder image upon said copy sheet.

3. A method according to claim 2 including fixing said rst powder image and fixing said second powder image.

4. A method of electrostatic printing comprising rst placing a thin layer of the powdered carbon black upon the upper surface of a layer of photoconductive insulating selenium supported upon a lower electrode and spaced from an upper electrode; then simultaneously applying a unidirectional electric lield of the order of 3200 volts per inch between said electrodes and projecting a light image upon said photoconductive insulating layer to charge the carbon black lying upon the illuminated areas of the layer of photoconductive insulating selenium and to attract the charged carbon black away from the layer of photoconductive insulating selenium leaving a rst visible powder image thereon.

5. A method according to claim 4 including intercepting the carbon black attracted away from said layer 20 structive Testing;

of photoconductive insulating selenium upon a paper copy sheet in closely spaced relation to said layer of photoconductive insulating selenium thereby producing a second visible powder image upon said copy sheet.

References Cited in the tile of this patent UNITED STATES PATENTS OTHER REFERENCES New Developments in Xeroradiography-Non-De Summer 1951; volume 10, No. 1, pages 8-25. Page 18 particularly relied upon. 

1. A METHOD OF ELECTROSTATIC PRINTING COMPRISING FIRST PLACING A THIN LAYER OF DEVELOPER POWDER UPON THE UPPER SURFACE OF A PHOTOCONDUCTIVE INSULATING LAYER SUPPORTED UPON A LOWER ELECTRODE AND SPACED FROM AN UPPER ELECTRODE; THEN SIMULTANEOUSLY APPLYING A UNIDIRECTIONAL ELECTRIC FIELD BETWEEN SAID ELECTRODES AND PROJECTING A LIGHT IMAGE UPON SAID PHOTOCONDUCTIVE INSULATING LAYER TO CHARGE THE DEVELOPER POWDER LYING UPON THE ILLUMINATED AREAS OF THE PHOTOCONDUCTIVE INSULATING LAYER AND TO ATTRACT THE CHARGED POWDER AWAY FROM THE PHOTOCONDUCTIVE INSULATING LAYER LEAVING A FIRST VISIBLE POWDER IMAGE THEREON. 